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Here some test results to evaluate the hardware performance and the use of a pseudo random binary sequence as reference to cross correlate the streams received from the 3radio, a radio composed by multiple RTL-SDR hardware.

The 3radio has been tuned to 69 MHz center frequency and a sampling rate of 2048000 Hz has been chosen. 3 streams of 100000 IQ sample have been recorded using librtlsdr . The real time length is half a second.

The same signal was routed via separated switching diodes to all the RTL-SDR receivers.

Sequences of about 3 ms or 6000 bits have been generated. A delay of some hundred of ms separates the different bursts. The reason was to be able to recognize some sync reference just looking to signals in time.

The previous figure shows the IQ streams in time, a chunk of about 6200 samples at 2048000 sampling is selected.

A script in python was written to compute the cross correlation. Here after the script:

# The raw, captured IQ data is 8 bit unsigned data.# Each I and Q value varies from 0 to 255.# To get from the unsigned (0 to 255) range we need to subtract 127.5# from each I and Q value, which results in a new range from -127.5 to +127.5.# The complex data is y = I + jQ and we subtract 127.5 +127.5j

The results look quite good nevertheless the reference signal was far from perfect as you can notice from the cross correlation side lobes.

The time differences of the max positions define the time skewing between the streams. This time depends on the USB latency, on the different starting time of the software application trigger. I hope that the hardware clock skewing is solved with the synchronization.